1. Field of the Invention
This invention relates to conductive polymer compositions and opto-electrical devices comprising conductive polymer compositions.
2. Related Technology
One class of opto-electrical devices is that using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light.
These devices have great potential for displays. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to optimise (e.g. to reduce) the voltage at which peak efficiency is obtained. Another is to stabilise the voltage characteristics of the device over time. Another is to increase the lifetime of the device.
To this end, numerous modifications have been made to the basic device structure described above in order to solve one or more of these problems. For example, a layer of a hole transport material may be provided between the anode and the light-emissive organic layer to assist transport of holes to the light-emissive organic layer. Also, a layer of an electron transport material may be provided between the cathode and the light-emissive organic layer to assist transport of electrons to the light-emissive organic layer.
Another such modification is the provision of a layer of conductive polymer between the light-emissive organic layer and one of the electrodes. It has been found that the provision of such a conductive polymer layer can improve the turn-on voltage, the brightness of the device at low voltage, the efficiency, the lifetime and the stability of the device.
Chem. Mater. 2004, 16, 708-716 discloses two conjugated fluorene polyelectrolytes (P2,P4), which are said to be suitable for use as electron injection layers.
One example of a suitable conductive polymer for use as a hole injection layer between the anode and the light-emissive organic layer is polystyrene sulfonic acid doped polyethylene dioxythiophene (“PEDOT-PSS”)—see EP 0,686,662. This composition provides an intermediate ionisation potential (intermediate between the ionisation potential of the anode and that of the emitter) a little above 4.8 eV, which helps the holes injected from the anode to reach the HOMO level of a material, such as an organic light emissive material or hole transporting material, in an adjacent layer of an opto-electrical device. The PEDOT-PSS may also contain epoxy-silane to produce cross-linking so as to provide a more robust layer. Typically the thickness of the PEDOT/PSS layer in a device is around 50 nm. The conductance of the layer is dependent on the thickness of the layer.
The chemical structures for PEDOT and PSS are shown below:

In a PEDOT-PSS composition the PEDOT is oxidized to produce a polymer radical cation which acts as a hole transporter. The oxidized dioxythiophene requires an anion to stabilise it; this is the PSS. The PSS ionises to produce a polymer anion which acts as a counter ion to stabilise the charge on the PEDOT.
PEDOT:PSS is water soluble and therefore solution processible. The provision of PEDOT:PSS between an ITO anode and an emissive layer (or a hole transport layer when present) increases hole injection from the ITO to the emissive layer, planarises the ITO anode surface, preventing local shorting currents and effectively makes energy difference for charge injection the same across the surface of the anode.
It has been found that varying the ratio of PEDOT:PSS in a layer of a device significantly changes the functional performance of the device.
A PEDOT:PSS ratio of 1:2.5 provides a stable processible solution. That is, materials with this ratio or higher PSS stay in solution. At low concentrations they come out of solution. However, at a ratio of 1:2.5 the conductivity is very high and as such this material cannot be used in some opto-electronic device arrangements as it can, for example, short connections between electrode lines in a device as discussed previously.
In practice, it has been found that using an excess of PSS (i.e. an amount in excess of the amount required to balance the charge on the PEDOT) can improve device performance and, in particular, can increase lifetime as disclosed in U.S. Pat. No. 6,605,823. Furthermore, excess PSS results in the composition being easier to ink jet print. By “excess PSS” is meant more PSS than is needed to prevent the PEDOT failing out of solution. Thus, using excess PSS, such as a PEDOT:PSS ratio of 1:6, 1:16 or even greater is useful in working devices.
It is evident from the above that it is advantageous to provide PSS in excess for ease of manufacture of a device and so as to produce a device with better performance and lifetime. However, there is always a desire to further improve the performance and lifetime of devices and make the manufacturing process easier and cheaper. Accordingly, alternatives to the PEDOT-PSS system having excess PSS are sought. Without being bound by theory, one possible limitation on the lifetime of devices using the aforementioned PEDOT-PSS system is that the provision of such a large excess of PSS results in a composition which is very acidic. This may cause several problems. For example, providing a high concentration of strong acid in contact with ITO may cause etching of the ITO with the release of indium, tin and oxygen components into the PEDOT which degrades the overlying light emitting polymer. Furthermore, the acid may interact with light emitting polymers resulting in charge separation which is detrimental to device performance.
Recently there have been studies of alternatives to PSS. For example, WO 04/029128 discloses the use of compositions comprising aqueous dispersions of polythiophenes and at least one colloid-forming polymeric acid as the buffer layer (hole injection layer) in an OLED. In particular a composition of PEDT/Nafion® (a fluorinated sulfonic acid) is disclosed.
Accordingly, there is a desire to provide an alternative to the aforementioned PEDOT-PSS system, preferably one which results in better device performance, lifetime and ease of manufacture.